In fluid circuits, such as hydraulic and pneumatic circuits, it is often desirable to maintain a constant pressure over a fluid device, such as a throttle valve or similar. Conventionally this has been achieved by providing a pressure controlling valve located in the fluid circuit before, or after, the fluid device. The pressure controlling valve is a pressure controlled valve that is controlled by the pressure drop across the fluid device. Pilot pressure conduits are connected upstream and downstream of the fluid device and the pilot pressures act on a spring loaded spool in the pressure controlling valve. The fluid device may be a throttle valve arranged to provide a set pressure drop. The spring load in the pressure controlling valve is selected so that the spool is maintained in a predetermined position. If the pressure drop increases or decreases, the pressure difference between the pilot pressure conduits will cause the spool to move to close or open the pressure controlling valve in order to maintain a constant pressure across the throttle valve.
This type of pressure controlling valve is arranged to control the pressure drop in one direction of flow through the fluid circuit. Recent developments in fluid circuit control have created a need for more energy efficient operation of fluid devices and circuits, including a need for recovering fluid pressure for regenerative purposes in order to minimize pressure and flow losses. Fluid circuits of this type may be provided with means for controlling fluid pressure in both directions. In order to achieve this, a circuit as described above must be provided with two pressure controlling valves, connected so that the valves may control the pressure drop in opposite directions of fluid flow.
One problem with this arrangement is that an additional valve is required for each such location in a fluid circuit, increasing the cost, complexity and weight of the fluid circuit.